A boiler is a closed vessel in which water or other fluid is heated under pressure to create steam. The ability of steam to carry large amounts of thermal energy is the property that makes it an effective working fluid. The generated steam is then circulated out of the boiler for use in various processes or heating applications. In a conventional boiler system, for example, water is heated under pressure in a boiler, creating steam that can be used to actuate a turbine, or used in a steam press. However, in such open systems much of the heat energy can be lost during the process.
In a boiler systems in general, when the properties (i.e. temperature, pressure, volume) of the working fluid are changed as a consequence of work or heat exchange, then it is said that the fluid has gone through a “process.” In some processes the relationship between pressure, temperature, and volume are specified as the fluid goes from one thermodynamic state to another. The most common processes are those in which the temperature, pressure, or volume is held constant during the process. If the fluid passes through various processes and then eventually returns to the same state it began with, the system is said to have undergone a cyclic process.
In a general cyclic boiler system, the thermal efficiency of the boiler can be increased by the recovery of waste heat. In such a closed loop system, after water enters a boiler where it is heated by an external heat source to become steam, the pressurized steam can be applied to a steam-powered device to perform some function, such as a steam-powered iron. This results in a decrease in the temperature and pressure of the steam vapor. To recover the waste heat, the vapor is collected in a condenser where it is cooled to become saturated liquid, i.e. warm feed water. This warm feed water is then pumped back into the boiler and the cycle is repeated. Because the waste heat from the steam vapor is recycled as warm feed water back into the boiler, the thermal efficiency of the system is improved.
One method of recovering waste heat is disclosed in U.S. Pat. No. 6,196,163 to Shah. In Shah, heat is recovered by capturing the hot condensate via a reactor steam jacket, then recycling the hot condensate back into the boiler. More specifically, high temperature steam condensate is collected from a reactor steam jacket into a closed vessel in which the pressure is that of the lowest pressure in the reactor steam jacket, but higher than atmospheric, such that there is little or no steam flash over due to a drop in pressure. The steam condensate is then fed back into the boiler. According to Shah, the amount of heat saving to the boiler is the amount of flash over steam saved and hotter condensate returned to the boiler by this method.
The invention described by Shah suffers from a number of disadvantages. For instance, an elaborate system of pumps is necessary for operation of the Shah method, which is not only more costly, but also consumes extra energy to operate and thus results in decreased overall efficiency of the system. More significantly, the boiler cycle described by Shah experiences significant heat loss while the condensate waits in the feed water tank to be fed back to the boiler.
Another method of recovering waste heat is disclosed in U.S. Pat. No. 7,331,312 to Choi. In Choi, the heat is recovered from the feed water in the feed water tank to heat water for a hot water tank. In other words, the Choi invention uses the heat in the boiler system that would be lost to heat water for a second system. Thus, the Choi invention does not increase the efficiency of the boiler system, but increase the efficiency of a second system. Though the Choi system utilize the heat of the feed water in the feed water tank waiting to be cycled back into the boiler, it does not utilize the heat from the low pressure steam in the feed water tank.
The present invention utilizes the heat from the low pressure steam in the feed water tank that forms as the high temperature feed water waits to be fed back to the boiler that would otherwise be lost as it dissipates to the environment to heat the make-up water source, e.g. ground city water. The make-up water is needed to replenish the water lost as the steam utilizing devices uses the steam to perform its process, such as a steam press using steam to iron clothes or other textile articles.
Additionally, the present invention may also utilize the heat that would be lost by the returning condensate and low pressure steam vapor returning to the feed water tank to heat the feed tank water, which otherwise would be lost through dissipation to the air as low-pressure steam. Unlike the Shah invention, which is directed to a method for recapture of lost heat by preventing hot condensate water from flashing through an elaborate set-up of sophisticated equipments, the present invention prevents hot condensate from flashing by shifting the excessive heat for use to heat the water in the feed tank and maintain that water in the feed tank at high temperatures.
Also unlike the Choi invention, the present invention is directed to utilizing the recaptured heat to increase the efficiency of the boiler system. Similarly, the present invention also utilizes the heat from the low pressure steam that is formed as the high temperature feed water in the feed water tank waits to be cycled back into the boiler system.
More generally, in systems wherein waste heat is typically recycled back into the boiler to improve the boiler efficiency, heat is still wasted at the stage when the process steam condenses to water and waits in the feed water tank to be pumped back into the boiler. While waiting in the feed water tank, heat dissipates into the environment. However, the heat energy that is lost from the hot condensate in the boiler system can be recaptured to heat the make-up water and to heat the feed water tank. In the present invention, the waste heat energy recovered in the form of hot condensate feed water can be used to heat the make-up water source and the feed water in the feed tank waiting to be recycled back to the steam boiler.
For the foregoing reasons, there is a need for a method and apparatus for recovering waste heat in a boiler system that can minimize the wasted heat energy in a boiler system.